ASM-1 and Tempcos

Joachim Verghese jocke at netcontrol.fi
Tue Jan 28 18:18:17 CET 1997


On Tue, 28 Jan 1997, Joe Farler wrote:

> So be warned: the Tempco's from Farnell have a negative coefficient

Thanks, that's valuable information. I assumed it was positive.
Big Ooops!

> On a slightly different note, anybody building the ASm-1 type osc as a 
> separate module please note that pins 1 and 4 on the 311 are 
> connected together.  I don't think this is shown on Gene's original
> schematic.

That's right - in this application we want to pull the FET gate
negative so as to block the channel. Therefore the emitter of
the 311's output transistor (pin 1) should go to -15V (together
with VEE, pin 4).

Also, it should be noted that the negative supply pin of the
CA3130 integrating op-amp should go to ground, not -15V. The 3130
cannot withstand more than 16V across its supply pins. (The CA3140
is suitable for +/-15V supplies).

BTW, the 3130/3140 probably *is* the best device for this particular
application. I've tried many other op-amps, but none of them gives
such a clean sawtooth. And, as Gene mentioned, you can have the VCO
oscillate at very low frequencies without switching in another
capacitor (although board layout becomes critical because of the
minimal control currents).

-joachim



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